The present invention relates to the art of laser light absorption. It finds particular application in conjunction with absorbing a laser beam directed at a moving vehicle, thereby decreasing the distance at which a speed detection device using a laser beam is effective. It is to be appreciated, however, that the invention may find application in other areas.
Numerous devices for detecting the speed of a traveling vehicle have been presented. Included among these is a system known as VASCAR, which is an acronym for Visual Average Speed Computer and Recorder. In its simplest form VASCAR is a calculator that determines a target car's speed from two factors, time and distance. With VASCAR a user implements two toggle switches to "clock" a car, one to start and stop a distance measurement and the other begins and ends the time measurement. The time basis is internal, while a sensor connected to the user's speedometer cable supplies distance information. A user will simply measure the length of the area over which the test will be done, which may be a span of roadway. Then, by flipping the switch twice, the user determines how long it takes a target car to cover the predetermined length. An internal calculator does the mathematical calculations and the average speed is displayed on a LED readout.
Another detection method uses radar (radio detection and ranging). A radar gun antenna transmits a microwave signal at the target vehicle, which reflects a portion of the wave back to the "gun". If both cars are moving at the same speed, the reflected wave will be the same frequency as this transmitted signal.
But if the target car is moving relative to the user's car, it will bounce waves back at a slightly different frequency, higher if the target is approaching, lower if the car is receding. The car's speed can be accurately determined by comparing the transmitted frequency of the outgoing beam with its echo. A microwave beam used in radar detection can be more than 200 feet wide at a distance of 2,000 feet, enough to cover several highway lanes, therefore, bringing into question which vehicle of a group of vehicles is being targeted.
A recent advancement in the detection of vehicle speeds is the use of a laser beam. Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. A laser beam will form only a 51/2 foot spot at a distance of 2,000 feet and can, therefore, be aimed directly at a car being targeted. A commonly used type of arrangement for a laser beam to detect moving automobiles is in the form of a "gun" which uses a gun site to target its beam of light on a specific car. Approximately every three milliseconds some laser "guns" can calculate how far away the car is based on the time it takes the beam of light to make a round trip. Speed is then determined by dividing the change in distance by the elapsed time between readings.
In the article "How To Beat Laser and Radar Guns" Car and Driver, November 1993, Vol. 39, page 124 (4), by Don Schroeder, operation of two commonly used laser or lidar guns are discussed. These include the Kustom Signals ProLaser and the LTI 20-20. Both use pulses of infrared laser light at approximately a 904-nanometer wavelength in order to measure speed. The gun fires each pulse at the target and will wait for its reflection to return. With these products the laser beam created by the pulses is very narrow, about four (4) feet wide at a thousand (1,000) feet. The pulses are also very short approximately thirty-five billionths of a second long and are fired very rapidly, as many three hundred and eighty-five (385) times a second for the ProLaser.
The above article notes that the LTI 20-20 can clock a car a half a mile away in about a one-third (1/2) of a second. Another related article "Do Laser Detectors Work?", Car and Driver, December 1992, pages 133-135, by Don Schroeder states tests done with the same devices were able to a clock a highly reflective car from a mile away.
Various methods and devices are discussed in these articles to detect or limit the effectiveness of laser guns. A commonly used device is a laser detector, an electronic device for detecting the presence of a laser beam. This is a highly complex piece of equipment which is mounted on the dash or grill of a car and attempts to warn a driver as to the use of a laser gun. In addition to laser detectors, the article "How To Beat Laser and Radar Guns" suggests that by turning on the lights of a vehicle, the effectiveness of a laser gun is diminished. This occurs since certain car lights emit not only visible light on the electromagnetic spectrum but also infrared light.
"How To Beat Laser and Radar Guns" also explained that removal of the license plates decreases the laser guns capabilities. As more and more cars are using plastic in their construction the license plate has become a prime reflective surface for the laser beam. In fact, literature regarding operation of the LTI 20-20 specifically instructs a user to measure the velocity of a vehicle by aiming the laser gun at the license plate area of the target vehicle. This operation is described in Laser Speed Detection System Operator's Manual LTI 20-20 from Laser Technology, Inc. 7399 South Tucson Way, Level B Englewood, Colo. 80112.
As previously discussed, "How To Beat Laser and Radar Guns" has found that emissions from car lights, especially high beams, was effective in lowering the useful range of laser guns. This article further noted, however, that by using additional lighting, interference with other drivers would likely occur. One way which was shown to eliminate this interference is to use a filter that absorbs the bright visible light but passes the invisible infrared light. The article concluded that by using a laser detector with the headlights and driving lights on, chances are significantly decreased that either laser gun described in the article, will receive an accurate first reading. Thus, a laser detector would then have time to sound an alarm prior to a useable reading, allowing the driver time to decelerate.
It should be noted that one of the most effective manners described in the above articles to decrease the useful range of a laser gun is removal of the license plate. However, since in all states both front and rear license plates or at least a rear license plate is required, this is not a practical solution.
The use of high-beams or powerful driving lights also has drawbacks including interference with other drivers. A suggestion to overcome this drawback is the use a filter that absorbs the bright visible light but passes the invisible infrared. Unfortunately, as also noted in the article, using such a filter would mean the light does not allow visible light to pass through. This would then require a user to remove the filters whenever visible light is desired, such as night driving. Another drawback with this solution is that the car lights would be used continuously, this would result in the lights burning out at a faster rate than would otherwise occur. Further, such constant use of the lights could put an undesirable drain on the vehicle's battery.
The present invention provides a new and improved license cover plate which will absorb laser light thereby decreasing the working range of a laser gun while overcoming the problems of other attempts to decrease the working range of a laser gun.